Solution with detailed explanation https://leetcode.com/problems/linked-list-random-node/
Linked List Random Node https://leetcode.com/problems/linked-list-random-node/
- Select first node with p = 1
- Select second node with p = 1/2
- Select kth node with p = 1/k
- Let P(k) be the probability of selecting kth node.
- Probability of kth node being selected after we have seen N elements (N>k) = P(k) * (1-P(k+1)) * (1-P(k+2)) * ... (1 - 1/N) = 1/N
from random import randint class Solution(object): def __init__(self, head): """ @param head The linked list's head. Note that the head is guanranteed to be not null, so it contains at least one node. :type head: ListNode """ self.head = head def getRandom(self): """ Returns a random node's value. :rtype: int """ rnode, count = None, 1 root = self.head while root: if randint(1, count) == 1: rnode = root root, count = root.next, count + 1 return rnode.val